Interactions between a hydrophobic organic chemical and natural organic matter: Equilibrium and kinetic studies

Interactions between a well-characterized, purified, peat humic acid and pentachlorobenzene were studied in equilibrium and kinetic experiments. The kinetic experiments, performed with a gas-purge method, showed the presence of a fast-desorbing, labile fraction and a slow-desorbing, nonlabile fraction. Increased contact time did not change the measured continuous desorption curves. A desorption experiment with interrupted flow and the measured isotherm suggested nonlinear sorption behavior. A first-order model consisting of two parallel "sites" could describe the continuous desorption curves but failed to describe the adsorption isotherm and the desorption curve with interrupted flow. A Langmuir model with one site was able to describe the characteristics of the desorption experiments and of the adsorption isotherm, but it did not describe the observed nonlabile fraction. Expanding the model with an additional Langmuir or first-order site did not lead to a good description of the various experiments. We speculate that the nonlabile fraction is a consequence of a change of conformation of the humic acid after the binding of pentachlorobenzene. This structural change leads to entrapment of pentachlorobenzene within the humic acid structure.